Many integrated circuits, such as RF amplifiers, generate a substantial amount of heat during operation. For example, contemporary RF amplifiers used in wireless communication systems often operate at temperatures approaching 200 degrees C. Clearly at such elevated temperatures, an efficient approach to dissipating the generated heat is required. Thus the packaging for these integrated circuits is generally formed on a heat sink made of a material e.g. copper containing or aluminum containing composition that has excellent heat conducting properties, and the packaging materials are chosen to be resistant to heat degradation.
Many such packages therefore are formed on a metal base or heat sink, 1, in FIG. 1 using materials such as alumina, 5, to form package walls that surround the integrated circuit, 3, forming a cavity package. The package walls provide mechanical and environmental protection to the integrated circuit. For hermetically sealed packages, a lid, typically metal or ceramic, is placed on top of the package after the integrated circuit is die bonded and wire bonded into the cavity region formed by the walls and subsequently sealed with a moisture impermeable material such as a metal or glass. For non-hermetically sealed packages the walls of the cavity package are used to form a dam for subsequent introduction of a polymer, 6, that encapsulates the integrated circuit. (The integrated circuit body is generally referred to as a die.) Electrical connection to the device, e.g. die is formed from metal leads, 7 and the base or heat sink 1. Wires are attached to the capacitor(s), 8, die(s) and leads to make electrical contact. Wire loops with precision shapes are used for proper electrical performance. The die and capacitors are bonded to the base to form thermal and electrical connection to the base. After forming the electrical interconnections the alumina walls, 9, are extended and an alumina cap, 10, is provided.
For many ceramic based packages the material employed for the heat sink is a composite of copper and tungsten. This metallic material is advantageous since it has a coefficient of thermal expansion approximately matching that of the overlying alumina walls. (The coefficient of thermal expansion for copper/tungsten ranges from 6.2 to 6.5 ppm/° C.(room temperature to 500° C.) as compared to approximately 6.9 to 7.2 ppm C.(room temperature to 400° C.) for alumina. Since the copper/tungsten alloy and the alumina have matching coefficients of thermal expansion, differential thermal expansion induced stresses at interfaces between the different materials is small so that the resulting cavity package is relatively stable despite large temperature excursions.
At the same time there has been a continuous drive toward higher and higher electrical power density per device to increase integration and decrease size. Therefore, to maintain a safe operating temperature, the power dissipation the package must provide increases. Accordingly, it becomes desirable to replace the copper/tungsten heat sink with a material that has superior heat conducting properties. One material that is low cost, readily available, easily manufactured in complex shapes, and has a high thermal conductivity is copper. Although copper has a heat conductivity of approximately 391 W/mK, (as compared to approximately 176 W/mK for copper/tungsten), its coefficient of thermal expansion, approximately 17 ppm/C. (room temperature), is a poor match for that of alumina. Thus the use of a copper heat sink despite its improved heat transfer characteristics is precluded for use with alumina walls, unless the copper is embedded into the center of a Cu-W base or some other base material that compensates sufficiently for the coefficient of thermal expansion of alumina. A composite Cu/Cu-W structure is significantly more expensive than a single Cu or Cu-W base. In addition such composite structure is more prone to deformation, and concomitant less than optimum thermal performance when mounted into the system.
To allow use of a copper heat sink, a polymer rather than alumina walls are employed. Polymers such as liquid crystal polymers have a coefficient of thermal expansion matching that of copper and have relatively high melting points compared to other polymers. Such polymers are commercially available from, for example, Ticona Manufacturing-Headquarters, 8040 Dixie Highway, Florence, Ky. 41042 U.S.A., Ticona, GmbH D-65926 Frankfurt am Main. In particular the Vectra line of materials have temperature stability up to 370° C. (Melting temperature (10° C./min); Test Standard: ISO 11357-1,-2,-3.) Although liquid crystal polymers have suitable thermal properties, their coefficient of thermal expansion is anisotropic. That is, their physical properties such as the coefficient of thermal expansion vary with orientation. In general for liquid crystal polymers, the thermal coefficient of expansion in the direction the polymer was drawn during preparation (parallel direction) is generally in the range 3 to 10 ppm/C. (0.03×10−4/° C. ISO 11359-2) while the coefficient of thermal expansion in a direction perpendicular to the draw direction (normal direction) is relatively large, 15 to 25 ppm/C. (0.19×10−4/°C. ISO 11359-2). Thus if the polymer forming the package walls is all aligned in the appropriate direction, an appropriate match to the thermal expansion properties of copper is possible. Unfortunately, typically at least a portion of the walls in the region adjoining the copper heat sink generally has the lower rather than higher coefficient of thermal expansion in a direction parallel with the major surface of the heat sink due to the requirements of the injection process used to form the walls. Thus although strain due to thermal mismatch between liquid crystal polymer walls and a copper heat sink is substantially reduced relative to a similar structure with alumina walls, thermal mismatch issues still remain.
Even once materials for the package are chosen, the assembly of the package using those materials is not free from difficulties. The height and shape of the lead wires, 4, in FIG. 1 are critical to tuning the RF response of the device. As the frequency of such devices increases, so does the height of these wire loops. Thus the height of walls, 5, must be extended so that the polymer, 6, introduced after the lead wires are connected, encapsulates such wires and prevents damage or a change in geometry.
FIG. 2 illustrates a wirebond tool head including wire clamps. The bonding tool is normally held vertically but wire clamps behind the tool are at an angle generally between 30 and 60 degrees. A tool shown at 22 in FIG. 2 is introduced, for example, to bond the wire, 21, to the capacitor block. Such bonding is accomplished by introducing ultrasonic energy through the tool together with compression also induced by the tool. As shown in FIG. 2, the height of the walls limits the angle at which it is possible to introduce the tool, 22. The geometry at the edge, 23, of the tool is chosen to accommodate this angle limitation. Generally an angle between 30 and 60 degrees has been employed. After the walls are positioned the wire bonds are formed and the remaining package is assembled.
Thus new packages employing copper heat sinks and polymer side walls have been introduced and solve many issues associated with high performance devices. However, improvement, as discussed, is certainly possible.